If there is a variance happening on a calabrated unit using the same sample of water it is usually a result of the optical flap not sitting properly.

Inspect the optical flap on the front of the unit, it should sit totally flat against the optic when at rest without having to push down. If it is sitting up a little one side the differing water thickness across the optics will cause the light to refract through the film sporadically giving different readings at each sample.

To adjust loosen the flaps collar with the included screw driver and adjust so the flap always comes to rest tight against the optic and doesn't sit up on one side.